MBTA Lost and Found: Depending on which subway line you thought you lost something, you can call the following numbers:
Blue: (617) 222-5533
Orange: (617) 222-5403
Green: (617) 222-5220
Red: (617) 222-5321
Silver: (617) 222-2432

Don't call the Lost and Found contact number found on the MBTA web site. It just leads to a phone directory with no obvious "lost and found" option. The MBTA lost and found is pretty good. A friend of mine accidentally left her Nintendo DS on the train yesterday while traveling to the airport. I called the Green Line lost and found this morning, and they had it, so I was able to pick it up for her.